Dzhokhar Tsarnaev admitted to authorities Sunday that he and his brother were behind the Boston Marathon bombings, according to a senior law enforcement official.

Tsarnaev made his admissions to FBI agents who interviewed him at Beth Israel Deaconess Medical Center, where he is being treated for multiple gunshot wounds. He had not yet been given a Miranda warning.

Tsarnaev’s attorneys are certain to challenge the legal admissibility of those admissions, and other information he gave them, such as claiming that he and his brothers acted alone, and that his brother was radicalized in an extreme form of Islam in part because he opposed US actions in Iraq and Afghanistan.

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But in an interview with the Globe, a senior police official said authorities are not worried about the initial admission to authorities being thrown out, because they have a strong witness: the man who was abducted by the Tsarnaev brothers last Thursday night.

Police sources told the Globe that the carjack victim has told police that Dzhokhar Tsarnaev and his older brother, Tamerlan, pointed guns at him and, in an apparent effort to intimidate the victim and dissuade him from trying anything foolish, Tamerlan Tsarnaev told him, “We just killed a cop. We blew up the marathon. And now we’re going to New York. Don’t [expletive] with us.”

The carjacking took place in Allston shortly after, police say, the Tsarnaevs ambushed Collier as he sat in his cruiser in Cambridge.

By the time Boston police, State Police, Watertown police, Transit Police and other officers confronted the Tsarnaevs early Friday morning after a Watertown officer spotted the stolen SUV, “we already knew these guys had admitted to killing three civilians and a police officer, and that they were prepared to kill many others,” the senior official said.

According to the official, the bombers repeatedly told the carjack victim that they were going to New York, which is why they used his ATM card at various locations: they needed cash for the trip.

Investigators are trying to determine if the brothers had either friends or co-conspirators in New York. But the haphazard, ill-planned escape has many investigators skeptical that there were other radical Islamists involved in the brothers’ attack.

“If they had accomplices in New York, you’d think they would have had an established contingency plan to get down there to them, and wouldn’t be shooting cops and carjacking cars to steal ATM cards to finance their escape,” the official said. “That said, we haven’t ruled out anything in New York. We’re looking into who they knew down there and was anyone in New York prepared to hide them.”